Electrochemical reduction treatment for soldering

ABSTRACT

The application of electrochemical reduction treatment liquids and electroless reduction treatment liquids to elements such as circuit boards for cleaning metallic surfaces in preparation for soldering is accomplished using a wave of the treatment liquid over a liquid tank. A wave treatment has advantages over spraying, which causes too much atomizing, and dipping, which is slow and requires complicated conveyors that are multi-directional. The method comprises forming a wave of reduction treatment liquid in an atmosphere having a limited included oxygen content and passing components through the wave so the reduction treatment liquid contacts the surfaces to be solder coated.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of co-pendingapplication 07/808,408 filed Dec. 16, 1991 Pat. No. 5,162,082.

1. Technical Field

The present invention relates to a method and apparatus for solderingusing electrochemical or electroless reduction. More specifically thepresent invention relates to utilizing a reduction treatment liquidapplied to surfaces to be soldered in order to remove by reduction theoxides formed on contaminated metallic surfaces to be solder coated orsolder joined.

2. Background Art

When applying a coating of solder to surfaces to be soldered, it isnecessary to remove oxides, otherwise wetting of the surface by soldermay not occur. The term "wetting" refers to solder bonding to metalsurfaces. If solder does not bond to metal surfaces then wetting has notoccurred. This is unacceptable, particularly on component leads to besolder coated and on printed wiring boards having components to besoldered thereon. In many cases, the components have pins that extendthrough holes and it is necessary to ensure that the solder "wicks" upthrough the holes and wets both the exterior surfaces of the pins andthe inside surfaces of the holes to ensure a proper soldered contact.

In the past one applied flux to the surfaces to be soldered and whereasflux dissolves the oxides that are present on the surface, flux andoxide residues may be formed in the process, and in the case of solderdipping or wave soldering, these residues can collect on the componentsor in the solder reservoir which can be harmful to the solder process,to the reliability of the electronic components or circuit assemblies,and in some cases to the environment.

It has been found that if soldering occurs on good solderable surfaceswithin an atmosphere with reduced oxygen and on occasions with a smallquantity of hot reducing gas such as hydrogen, that the need for flux isreduced and in certain cases not needed at all. Good wetting occurs andreliable soldering can result. However, oxide deposits that have builtup on metal surfaces require to be removed prior to soldering to ensureproper solder wetting.

In a paper published in June, 1987, entitled "New Copper SurfaceTreatment for Polyimide Multilayer Printed Wiring Boards" by HaruoAkahoshi and Yoshihiro Suzuki, is disclosed a reduction treatment for asurface oxide layer to give a metallic copper surface with no changes inits morphology. This conversion was shown to occur by means of wetchemical or electrochemical reduction. Varying the current density inthe electrochemical reaction changed the surface structure. It wasreported that a higher current density retained the original surfacestructure while low current density changed the original surfacestructure. The authors reported that if the original surface structureis retained, the adhesion characteristics are identical to those of theuntreated surfaces.

Electrochemical reduction for copper and tin-lead surfaces is disclosedin an article by Tench and Anderson entitled "Electrochemical Assessmentof Sn-Pb Solderability" published August, 1990, in Plating and SurfaceFinishing. The electrochemical reduction was performed in a sodiumborate solution under an inert gas utilizing a Pt counter electrode anda reference saturated calomel electrode (SCE).

Reduction of the oxides on metallic surfaces for improvements inmultilayer board adhesion using electrochemical reduction is disclosedin the above article. Unlike chemical treatment which uses a flux todissolve oxides, electrochemical reduction treatment does not dissolvethe oxides but treats the surfaces electrochemically. The elementshaving metallic oxides on the surfaces to be electrochemically treatedhave the liquid applied to these surfaces, usually in a dip tanksomewhat similar to an electroplating bath. The liquid reduces theoxides wherein the treated metal surfaces are cleaned of oxides.

Electrochemical reduction, sometimes known as reverse plating hasconventionally been a batch type operation such as dipping in a tankcontaining treatment liquid. Spraying, while used for wet chemicalspraying, generally is not appropriate with wet batch processing asapplied in board fabrication shops. Spraying cannot be used forelectrochemical reduction as current will not flow in a spray, unless acontinuous flood is used. Continuous dipping requires a conveyor whichtravels down into a tank and then up out of the tank. Such a non-in-lineconveyor becomes complex. Thus there is a requirement to floodcomponents such as circuit boards and the like which is simple and doesnot cause atomization of the treatment liquid.

DISCLOSURE OF INVENTION

It is an aim of the present invention to provide a novel method andapparatus for generously flooding an electrochemical liquid, or in somecases an electroless liquid, onto metallic surfaces of an element to betreated for soldering to remove oxides by reduction rather than byfluxing, without dipping or spraying.

The present invention provides in a method of treating metallic surfacesof components to be solder coated or joined, including the steps ofapplying a reduction treatment liquid to reduce oxides on the surfacesof the components within an atmosphere having a limited included oxygencontent prior to solder coating or joining, the improvement comprisingthe steps of forming a wave of reduction treatment liquid in theatmosphere, and passing the components through the wave so the reductiontreatment liquid contacts the surfaces to be solder coated or joined.

In a further embodiment there is provided an apparatus for applyingelectrochemical reduction treatment liquid or an electroless reductiontreatment liquid to metallic surfaces of components to be solder coatedor joined, comprising a nozzle adapted to form a wave of the reductiontreatment liquid, a pump means for pumping the reduction treatmentliquid to form the wave, conveying means for conveying componentsthrough the wave so the surfaces to be solder coated or joined arecontacted by the reduction treatment liquid, and means to provide anatmosphere having a limited included oxygen content in the region of thenozzle. In another embodiment, a rinsing stage occurs after the surfacesare contacted by the reduction treatment liquid.

BRIEF DESCRIPTION OF DRAWINGS

In drawings which illustrate embodiments of the present invention:

FIG. 1 is a schematic elevational view showing a reduction treatmentliquid wave in a first chamber, followed by a solder wave in a secondchamber;

FIG. 2 is a schematic elevational view showing a treatment liquid wavearrangement according to one embodiment of the present invention;

FIG. 3 is a schematic elevational view showing a treatment liquid wavein a chamber followed by wave soldering;

FIG. 4 is a schematic elevational view showing a reduction treatmentliquid wave in a first chamber, followed by a second chamber wheresolder coating occurs;

FIG. 5 is a schematic elevational view similar to that shown in FIG. 4having gas curtains between a treatment chamber and a soldering chamber;

FIG. 6 is a schematic elevational view showing drip trays on each sideof a tank with a treatment liquid wave arrangement therein.

FIG. 7 is a schematic elevational view showing an intermediate rinsingchamber and drying stage between a reduction treatment chamber and asolder wave chamber.

BEST MODES FOR CARRYING OUT THE INVENTION

FIG. 1 shows schematically a reduction treatment chamber 10 followed bya soldering chamber 12. A conveyor 14 transports components 16 firstinto the treatment chamber 10 and then into the soldering chamber 12.The treatment chamber 10 has an atmosphere with a limited includedoxygen content to prevent further oxidizing occurring on the metalsurfaces to be treated for solder coating or joining. A component 16enters the treatment chamber 10 through a flap, a gate, door, gascurtain, or other opening that tends not to permit air to enter. In oneembodiment, for instance, a positive pressure is attained within thetreatment chamber 10 thus ensuring that whereas some gas escapes throughthe entrance and exit, the atmosphere within the chamber 10 has alimited included oxygen content.

To control oxygen content in the atmosphere of the treatment chamber 10and the soldering chamber 12, it is preferred to add nitrogen. Totalexclusion of oxygen may be desirable but becomes expensive due to thecost of "pure" nitrogen. Cryogenic nitrogen has an oxygen impurity levelof about 2 ppm oxygen but the use of this purity of nitrogen provides acostly process that would be required for high reliability applicationsor for metal surfaces which would otherwise readily become reoxidizedagain.

In other applications a limited amount of oxygen or air is provided withnitrogen or other suitable types of inerting gas and fed to thetreatment chamber 10 and/or the soldering chamber 12. The inert gas andoxygen is provided at the input to the chambers, for example by the useof membrane technology where compressed air is fed through a membrane ora series of membranes to separate oxygen from nitrogen. Such technologycan be used to provide nitrogen with residual oxygen varying from as lowas 100 ppm up to for example 20% oxygen, just below the oxygen contentof air.

Thus the atmosphere in the chambers is controlled to restrict oxygencontent below that of air and in certain situations down tosubstantially no oxygen, i.e., approximately 5 to 10 ppm. In a preferredembodiment the atmosphere in the chambers excludes oxygen to a level of5 to 5000 ppm residual oxygen.

A component 16 passes through a reduction treatment liquid wave 18 whichextends up from a nozzle 20 and drops back into a reservoir 22 havingdrip trays 24 on both sides. The treatment liquid wave 18 eithercontacts only the bottom surface of the component 16 or, alternatively,floods over the top surface as well as the bottom surface of thecomponent 16. This can be varied depending on the requirements forcomponents to be soldered. For instance, if soldering is to occur onboth the top surface and the bottom surface, then it may in some casesbe necessary to provide the electrochemical reduction treatment liquidor electroless reduction treatment liquid to both surfaces of thecomponents.

After passing through the treatment liquid wave 18, the component 16passes from the reduction treatment chamber 10 into the solderingchamber 12, through a solder wave 26 from a solder reservoir 28 andexits from the soldering chamber 12. In one embodiment oxygen is limitedin the soldering chamber 12 as in the treatment chamber 10. Theexclusion of oxygen may be more, less or the same in the solderingchamber 12 as in the treatment chamber 10 depending upon therequirements of different components 16 being treated. Whereas thesoldering chamber 12 is shown adjacent to and in the same conveyor line14 as the reduction treatment chamber 10, it will be apparent that incertain circumstances the reduction treatment chamber 10 may be a batchtype chamber wherein components are passed in through a doorway, a dooror flap closed, treated in the liquid wave 18, and then removed aftertreatment through the doorway. The treated components may then be keptfor at least 24 to 48 hours prior to being soldered. This time periodmay be limited or reduced as required by deterioration of the treatedsurfaces to be soldered due at least partly to the environment andatmospheric conditions in the location where the components are stored.The time period may also be reduced to meet production requirements.

A reduction treatment tank 22 is illustrated in FIG. 2 wherein a pump 30is shown pumping electrochemical or electroless reduction treatmentliquid through the nozzle 20 to form the reduction treatment wave 18.The component 16 in the form of a board has a series of pins 32 in holes34 and the reduction treatment liquid wicks up between the pins 32 andthe holes 34 to ensure that metal oxides on the surfaces within theholes 34 and the pins 32 are treated so that complete solder wetting canoccur. The treatment liquid also treats the pads on the top side of aboard where applicable. In one embodiment, a vibrator vane 36 ispositioned in the nozzle 20 having an attachment arm 38 to a vibrator40. The vibration range is below ultrasonic frequencies, and preferablyin the range of about 20 to 400 Hz. The vibrator 40 oscillates the vane36 and in some cases it is found that such a vibration in the wave 18provides better wicking up of the reduction treatment liquid into thespace between the pins 32 and the holes 34 of the component 16 and outonto the pads on the top sides of the boards.

FIG. 1 illustrates the conveyor 14 conveying components 16 in asubstantially horizontal path through the treatment chamber 10 followedby the soldering chamber 12. In other embodiments the path may slopeupwards, or may be horizontal in the treatment chamber 10 and slopeupwards in the soldering chamber 12.

As shown in FIG. 2 a deflector plate 41 is positioned between the sideof the nozzle 20 and the tank 22, and a circulating pipe above the plate41 circulates the reduction treatment liquid through a pump 42 whichpasses it through a refresher 44 to remove metal, oxides, and any otherdeposits which have been removed by the treatment liquid from theboards, and returns a clean reduction treatment liquid to the tank 22 sothat the pump 30 pumps refreshed reduction treatment liquid to the wave18.

In one embodiment as shown in FIG. 1, the soldering chamber 12 isenclosed and has an atmosphere with a limited amount of included oxygentherein so that the soldering occurs within an atmosphere that lessensthe possibility of re-oxidation of metal surfaces. Seals may be providedat entry and exit to the soldering chamber 12. FIG. 3 illustratesanother embodiment wherein the reduction treatment occurs in a reductiontreatment chamber 10 having an atmosphere with limited oxygen therein,followed by soldering in a solder wave 26 which is not enclosed in achamber. In this embodiment, the conveyor line 14 slopes upwards toassist in draining liquid off the components. FIG. 4 illustrates areduction treatment chamber 10 and the soldering chamber 12 side-by-sidewith flaps 46 at the entrance and exit to each chamber 10, 12. Thesoldering may be reflow soldering or other known types of soldering suchas infrared, vapour phase, laser, ultrasonic. A preheating zone may beutilized within the soldering chamber with heaters 50 positioned belowthe conveyor line 14 as shown in FIG. 1. FIG. 3 illustrates heaters 53positioned above and below the conveyor line 14 before the solder wave.The heaters may be radiant or convection type, or combinations of thesetypes. As stated the soldering chamber may be filled with an inert gassuch as nitrogen.

FIG. 5 illustrates a further embodiment wherein gas curtains 48 areshown before the reduction treatment chamber 10, between the reductiontreatment chamber 10 and the soldering chamber 12, and after thesoldering chamber 12. The reduction treatment chamber 10 in oneembodiment includes a blow-off stage with gas knives 64 positioned toblow the liquid off the components after they pass through the reductiontreatment liquid wave 18. The gas knives 64 are shown staggered with thefirst gas knife 64 located below the conveyor line 14 followed by thesecond gas knife 64 located above the conveyor line 14. Otherarrangements of gas knives 64 may be provided. In one embodiment, thegas curtain 48 at the exit of the reduction treatment chamber 10 may beused in place of the air knives 64 to assist in blowing off any liquidthat remains on the components. Whereas air knives are shown, othertypes of gas jet or jets may perform the same function.

FIG. 6 illustrates a large component 16 which extends beyond the lengthof the reduction treatment liquid wave 18. As illustrated here the wave18 applies the reduction treatment liquid to the top and bottom surfacesof the component 16 thus the liquid falls off the component 16 on bothends and large drip trays 24 are provided to ensure that the liquidreturns to the tank 22.

FIG. 7 shows a washing and drying stage provided between the treatmentchamber 10 and the soldering chamber 12. An intermediate rinsing chamber54 is located next to the treatment chamber 10, and components 16 on theconveyor 14 enter the rinsing chamber 54 through a door flap or gascurtain (not shown). A rinsing wave 56 extends up from a nozzle 58 tothoroughly rinse the component 16 with liquid from a pump 59 and dropsback into a liquid tank 60. The rinsing liquid may be water or asuitable solvent for removing the treatment liquid applied in thetreatment chamber 10.

Any remaining rinsing liquid on the components is removed in a dryingzone 62 by jets 64. The jets 64 may use air or a gas with a limitedoxygen content, heat may also be applied to the drying gas. After thedrying zone 62, the component 16 on the conveyor 14 enters the solderingchamber 12. Drying of the components 16 continues as they are conveyedinto the soldering chamber 12 past the preheaters 50. The heaters may bepositioned both above and below as shown in FIG. 3. They may be infraredheaters, high volume convection type heaters, or other suitable heaters.

The rinsing chamber 54 may have an atmosphere with a limited oxygencontent as may the drying zone 62 to restrict oxidation of componentsurfaces prior to soldering.

FIG. 1 shows a solder wave 26 in a soldering chamber 12, and FIG. 3shows a solder wave 26 not in a soldering chamber. FIG. 4 shows asoldering chamber. The solder coating or joining can be applied by knownsoldering devices and methods which include different types of solderwave or multiple solder waves, vapour phase soldering, reflow solderingand other devices and methods available for solder coating and joining.In another embodiment, reduction treatment of the components occursaccording to the methods described herein, the treated components arethen stored in an adequate storage chamber which in certain cases isinerted. The soldering step is carried out sometime later, when it isappropriate in the production process.

Various changes may be made to the embodiments shown and describedherein without departing from the scope of the present invention whichis limited only by the following claims.

The embodiments of the present invention in which an exclusive propertyor privilege is claimed are defined as follows:
 1. In a method oftreating metallic surfaces of components to be solder coated or joined,including the steps of applying a reduction treatment liquid to reduceoxides on the surfaces of the components within an atmosphere having alimited included oxygen content prior to solder coating or joining, theimprovement comprising the steps of:forming a wave of reductiontreatment liquid in the atmosphere, and passing the components throughthe wave so the reduction treatment liquid contacts the surfaces to besolder coated or joined.
 2. The method of treating components accordingto claim 1 wherein the atmosphere is within a chamber and has an oxygencontent less than 20%.
 3. The method of treating components according toclaim 1 wherein the atmosphere within the chamber is nitrogen having anoxygen content in the range of about 5 to 5000 ppm.
 4. The method oftreating components according to claim 2 wherein the chamber is a closedchamber having closure means at least at one access for insertion ofcomponents.
 5. The method of treating components according to claim 2including a conveyor to transport components through the chamber andincluding a closure means at an entry and an exit to maintain theatmosphere within the chamber.
 6. The method of treating componentsaccording to claim 5 including gas curtains at entrance and exit, theexit curtains adapted to blow off reduction treatment liquid remainingon the components.
 7. The method of treating components according toclaim 1 wherein the wave of reduction treatment liquid is formed by pumpmeans pumping the liquid up through a nozzle and including a means torefresh the liquid before being formed into the wave.
 8. The method oftreating components according to claim 1 including a vibrator to vibratereduction treatment liquid in the wave, the vibrator adapted tooscillate a vane in the wave in the range of about 20 to 400 Hz.
 9. Themethod of treating components according to claim 1 wherein the reductiontreatment liquid is an electrochemical reduction treatment liquid or anelectroless reduction treatment liquid that reduces oxides on metalsurfaces.
 10. The method of treating components according to claim 1wherein any remaining reduction treatment liquid on the components isremoved by blowing after treating.
 11. The method of treating componentsaccording to claim 5 wherein the conveyor is inclined to slope upwardsin a direction of travel.
 12. The method of treating componentsaccording to claim 5 wherein the conveyor is substantially horizontal.13. The method of treating components according to claim 1 including thestep of rinsing the surfaces to be solder coated or joined after beingcontacted with the reduction treatment liquid.
 14. The method oftreating components according to claim 1 including the additional stepof solder coating or solder joining the metallic surfaces of thecomponents after being treated.
 15. An apparatus for applying anelectrochemical reduction treatment liquid or an electroless reductiontreatment liquid to metallic surfaces of components to be solder coatedor joined comprising:a nozzle adapted to form a wave of the reductiontreatment liquid; a pump means for pumping the reduction treatmentliquid to form the wave; conveying means for conveying componentsthrough the wave so the surfaces to be solder coated or joined arecontacted by the reduction treatment liquid, and means to provide anatmosphere having a limited included oxygen content in the region of thenozzle.
 16. The apparatus for applying a reduction treatment liquidaccording to claim 15 wherein the nozzle is in a chamber having closuremeans at an entry and exit.
 17. The apparatus for applying a reductiontreatment liquid according to claim 15 wherein a vibrator oscillates avane in the wave of reduction treatment liquid in the range of about 20to 400 Hz.
 18. The apparatus for applying a reduction treatment liquidaccording to claim 15 including a refresher means for refreshing thereduction treatment liquid prior to pumping the liquid to form the wave.19. The apparatus for applying a reduction treatment liquid according toclaim 16 wherein the closure means comprise gas curtains.
 20. Theapparatus for applying a reduction treatment liquid according to claim16 including drying means to blow off reduction treatment liquidremaining on the surfaces of the component after passing through thewave.
 21. The apparatus for applying a reduction treatment liquidaccording to claim 15 including drip trays on each side of the nozzle toensure reduction treatment liquid flowing over top surfaces of thecomponent fall back into a tank from which the wave of reductiontreatment liquid is pumped.
 22. The apparatus for applying a reductiontreatment liquid according to claim 15 wherein the conveyor means isinclined to slope upwards in a direction of travel.
 23. The apparatusfor applying a reduction treatment liquid according to claim 15 whereinthe conveyor means is substantially horizontal.
 24. The apparatus forapplying a reduction treatment liquid according to claim 15 includingsoldering means provided to solder coat or solder join the metallicsurfaces of the components after being treated.
 25. The apparatus forapplying a reduction treatment liquid according to claim 24 wherein thesolder means comprises a solder wave.
 26. The apparatus for applying areduction treatment liquid according to claim 15 including a rinsingmeans for rinsing the components on the conveyor after being contactedby the reduction treatment liquid.
 27. The apparatus for applying areduction treatment liquid according to claim 16 including a rinsingchamber adjacent the chamber for applying a reduction treatment liquid,the rinsing chamber having rinsing means therein for rinsing thecomponents on the conveyor after being contacted by the reductiontreatment liquid.
 28. The apparatus for applying a reduction treatmentliquid according to claim 27 wherein the rinsing means includes a nozzlewithin the rinsing chamber and pump means for pumping rinsing liquid ina wave to contact the components.
 29. The apparatus for applying areduction treatment liquid according to claim 27 including a drying zonelocated after the rinsing chamber.
 30. The apparatus for applying areduction treatment liquid according to claim 24 including a preheatingzone after the components have been treated and prior to the solderingmeans.
 31. The apparatus for applying a reduction treatment liquidaccording to claim 30 wherein at least one heater is positioned belowthe conveying means.
 32. The apparatus for applying a reductiontreatment liquid according to claim 30 wherein at least one heater ispositioned below the conveying means and at least one heater ispositioned above the conveyor means.
 33. The apparatus for applying areduction treatment liquid according to claim 20 wherein the dryingmeans to blow off reduction treatment liquid remaining on the surfacesof the components after passing through the wave comprises a first gasknife positioned below the conveying means followed by a second gasknife positioned above the conveying means.